Gas-Liquid Sensible Heat Transfer in Spray and Packed Bed under a Centrifugal Field Abhijit Mondal, Avijit Bhowal,* and Siddhartha Datta Department of Chemical Engineering, Jadavpur University, Kolkata 700032, India ABSTRACT: Sensible heat transfer rates between a heated liquid (dibutyl phthalate) and an air stream in direct contact have been studied in different contactor configurations (spray, spray with liquid redistribution, and packed bed) wherein the liquid flows under the influence of centrifugal force rather than terrestrial gravity as in traditional equipments. Experiments were carried out by contacting the phases counter-currently between two coaxial circular rotating disks. The cooling range of the liquid in these contactors was obtained by varying mass flux of liquid and air, and the rotational speed between 0.35 and 0.67 kg/m 2 ·s, 0.26 and 0.52 kg/m 2 ·s, and 300 and 900 rpm, respectively. The cooling range in the spray mode of operation increased with rotational speed. The ratio of the cooling range obtained in rotating packed bed and spray operation decreased at higher rotational speed (other conditions remaining same). Below a critical rotational speed, the cooling range in spray operation could be further extended by redistributing the liquid in the spray zone. The overall volumetric heat transfer coefficients in these three contactors determined by simple mathematical models monotonically increased with centrifugal acceleration and were between 5 and 10 times higher than in traditional ones. ■ INTRODUCTION Direct contact heat exchangers involve heat transfer between hot and cold streams of two phases in the absence of a separating wall. The process offers an attractive approach for energy recovery as compared to conventional heat exchangers because of manifold advantages such as higher effective heat- transfer coefficients, absence of surface scaling, operation at low temperature differentials, among others. Existing technologies for direct contact gas-liquid heat exchange rely largely on spray columns, and columns with packing or trays. The operation is carried out in a vertical vessel. The liquid flows downward in these equipments under the influence of terrestrial gravity. Data involving purely sensible heat transfer for air-liquid system are not abundant. Fair 1,2 proposed design correlations for several direct contact heat exchange devices such as packed column, sieve tray, and spray column. Spiegel et al. 3 reported that overall heat transfer coefficient in packed (Mellapak 250.X) bed varied between 20 and 50 W/m 2 ·K (liquid flow rate 3-25 m 3 /m 2 ·h, air F-factor 0.9-2.2 m/s(kg/m 3 ) 0.5 ) for the dibutyl phthalate/air system. Designing efficient equipments is one of the most promising challenges for this process. Bruckner and Mattick 4 conceptual- ized a direct contact liquid drop/gas heat exchanger for thermal management in space wherein a vortex chamber served the dual purpose of heat exchanger and separation of the fine droplets from the gas stream. Hattori et al. 5 theoretically examined the thermal efficiency of a device in which the liquid flowed down wires suspended in the gaseous stream. The thermal energy recovery was shown to be higher than that accomplished by direct spraying of the liquid. Several researchers 6,7 have studied the breakup of liquid jets from a rotating orifice. Drops produced were noted to become smaller with the increase of the rotational speed. The gas- liquid heat transfer coefficient increases with decrease of drop diameter according to available correlations. 8 In recent years, rotating packed bed operating at hundreds of times the terrestrial gravity has been exploited 9-11 to reduce liquid film thickness, and permit use of high surface area packing for intensifying mass transfer rates in gas-liquid systems. The studies indicate that higher values of volumetric coefficients could be achieved and equipment size reduced for direct contact gas-liquid heat transfer as compared to traditional spray and packed bed contactors, if the liquid flow is dictated by centrifugal force. In view of the above-mentioned possibility of intensification in heat transfer rates, it is necessary to obtain comprehensive information on direct contact gas-liquid heat transfer rates achievable under centrifugal acceleration for potential industrial applications. However, no attempt appears to have been made in this direction. The objective of the present study was (i) examining the effect of rotational speed on the sensible heat transfer characteristics, and (ii) determining the volumetric heat transfer coefficient, in different contactor configurations. ■ EXPERIMENTAL SECTION The system selected for the study was air/dibutylpthalate (DBP). The liquid has a very low vapor pressure, and hence the data can be used for directly evaluating the volumetric heat transfer coefficients. 3 A general diagram of the experimental setup is shown in Figure 1. Heat transfer studies between dibutyl phthalate (DBP) and air flowing counter-currently were carried out between two coaxial stainless steel disks. Each of the disks was rotated by an AC motor through a shaft. The outer diameter of the disks and the distance between them were 0.32 and 0.03 m, respectively. The diameter of the cylindrical shaped stationary Received: April 30, 2012 Revised: December 5, 2012 Accepted: December 12, 2012 Published: December 12, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 499 dx.doi.org/10.1021/ie301116s | Ind. Eng. Chem. Res. 2013, 52, 499-506